Cooking utensils comprising a nonstick coating with a fluorocarbon resin base (in particular with a PTFE base) are traditionally favored by the market because they make possible a baking without (or practically without) added greases while still remaining easy to clean. However, such utensils have the disadvantage of a low resistance to scratches. Yet, the resistance to scratches is a property that is difficult to characterize as it depends on several other properties such as the intrinsic hardness of the material, the elasticity, the resistance to abrasion, and the coefficient of friction. As it is not only difficult to measure these properties separately, but also in combination, this is limited in practice to abrasion tests that reveal only imperfectly the cooking reality and to actual kitchen tests.
In order to improve the resistance to scratches of nonstick coatings with a fluorocarbon resin base (in particular with a PTFE base), it is known to those skilled in the art to reinforce the nonstick coating with hard fillers (in particular primers reinforced with hard fillers) or by the interposition of a hard base of the inorganic type between the support of the cooking utensil (generally metal) and the nonstick coating.
The reinforced primers effectively make it possible to substantially improve the resistance to abrasion. But impacts to the metal during the cooking of food such as pork ribs, or during the use of metal spatulas, are also observed.
The inorganic hard bases, such as for example those made from a glaze, make it possible to further improve the resistance to abrasion. Furthermore, the problem of impacts is practically eliminated with such bases. However, new brittleness is introduced, as for example a certain sensitivity to hydrolysis, and the quasi-impossibility to add bottoms via a strike (in order to obtain so-called struck bottoms for utensils compatible with an induction heating, constituted of an aluminum bowl and of a grid made of ferritic stainless steel. The strike allows for the bonding of the grid on the outer surface of the bottom of the bowl).
The resistance to hydrolysis is required due to the porosity of the nonstick coating with a fluorocarbon resin (PTFE) base, in light of the necessity of good resistance in the dishwasher. Yet, a good bonding of the inorganic hard base to the support requires a substantial share of flux, which negatively impacts the resistance to hydrolysis. The use of a hard base having very good properties of adherence to the support (so-called “super-adherent” hard base) is therefore carried out to the detriment of the resistance to the hydrolysis.
It then follows that the use of a hard base with glaze typically remains confined to the reinforcement of utensils that are already formed, in other terms utensils that do not undergo any deformation after the deposit of the hard base. As such, a hard base with glaze cannot be used to reinforce utensils made from a disk for which the formation is carried out after the deposit of the nonstick coating. Another disadvantage of this way of proceeding is the high energy cost of this technology. Indeed, the baking of a hard base with glaze requires a heat treatment of a magnitude of 560° C. for several minutes. This treatment does not pose any problem when the outer surface of the bowls is coated with a glaze, as the baking of the inner glaze (hard base) and outer glaze (decoration) can then be carried out simultaneously. This is not the case if it is desired to coat the outer surface of the utensil with a coating with a PTFE base, which is generally done with a single passage in the oven. The baking of the hard base then imposes an additional passage, which is very costly in terms of energy by the temperatures and the durations of the baking that are required.
Moreover, in the case of multilayer utensils incorporating at least one aluminum layer or in the case of utensils made of stainless steel incorporating an added bottom constituted of ferritic stainless steel and of aluminum, the hard base with glaze, which is in contact with the stainless steel, must have a high softening point that is higher than the melting point of the aluminum. It then follows that the baking (typically at a temperature of at least 800° C.) of this hard base results in the detaching of the various parts of the utensil. A hard base with glaze therefore cannot be used with multilayer products.
For the reasons mentioned above, hard bases with glaze are therefore far from providing a response that is entirely adapted.
Finally, those skilled in the art also know metal or ceramic hard bases applied via plasma or par thermal spraying on a metal support. Typically, using an electric arc, a plasma torch or a flame, a hard base is applied in the form of a continuous layer which covers the entire surface before the coating of the PTFE. These metal or ceramic hard bases are continuous and are generally very thick: they can therefore be applied only for utensils that are already formed. Indeed, the presence of such thick and continuous bases under the nonstick coating prevents any later stamping or forming of the utensils, which prohibits them from being used on flat disks. Moreover, as it is sought to create the smoothest surface possible in order to avoid disturbing the adhesion and the nonstick properties of the coating, it then follows that the application of such bases requires a long time to implement and a succession of tools and layers to create this smooth surface.
Finally, the presence of a ceramic layer over the entire utensil also creates problems during the finishing of the utensil, in particular the operation of trimming is made more expensive by the requirement of special tools and less productivity.
These ceramic and/or metal continuous bases therefore also have disadvantages.